Locating part on a movable jaw?

I'm working for a company that uses some sketchy methods to produce it's parts. Currently we are running a round part with two flanges on one side and those flanges are placed on pins in machined soft jaws on the front jaw of a vice. I have always been told locating on the front moving jaw is a little improper. I have ask dozens of times for the reasoning for this method but haven't ever gotten a response. I would hear your guys advice on this. Is it true you shouldn't locate your part on the front jaw? Is there a good reason for doing it this way?

When you reference a part off a fixed jaw, you know where the part is in relation to the table. If the pins are on the moveable jaw, any variations in part size will affect the location of the pins/ flange holes with respect to the table. It may not matter if you indicate a known feature of the part every time, but you lose the advantage of fixturing


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I'm definitely aware of this, it tracks with what I've been taught over the years. They aren't indicating anything, it's a load and go kind of situation as far as I can tell. They might be probing it, not sure if it's still in the program. I'm just wondering if there is any advantage gained by location the part this way? We always have trouble with this job. I hate seeing it on the schedule, as a shop supervisor I always have to pay close attention to while we run it.

If your the boss, then fix the problem.

Ben117 said:

I'm working for a company that uses some sketchy methods to produce it's parts. Currently we are running a round part with two flanges on one side and those flanges are placed on pins in machined soft jaws on the front jaw of a vice. I have always been told locating on the front moving jaw is a little improper. I have ask dozens of times for the reasoning for this method but haven't ever gotten a response. I would hear your guys advice on this. Is it true you shouldn't locate your part on the front jaw? Is there a good reason for doing it this way?

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There could be a few reasons. but depending on what the print call out is, how its cut and what the dims are called from. there could be a reason.
with out seeing a print or location points and the set up they could be correct.

Really wish I could. Not the boss, just the baby sitter. I have an Ops manager and VP of ops who won't change anything period. I even reminded them they fired the programmer for incompetence. Still won't budge! I was hoping launching a forum with enough information could possibly change their minds on this particular job.

Can't give a print either, aerospace. Can't even snap a pic with my phone. No phones, cameras everywhere. Pretty sure all the GD&T comes off the main structure of the part though. Think Micky mouse if you lay the part on it's side. If you could give me the reasons you are thinking of I could check it to my print. Thanks for all the input so far, sorry for my lack of information.

You're asking for validation, for your perception of incorrect methods. It's too hard to give you that, without more information. Generally I don't use the front jaw, BUT I have.

Ben117 said:

Can't give a print either, aerospace. Can't even snap a pic with my phone. No phones, cameras everywhere. Pretty sure all the GD&T comes off the main structure of the part though. Think Micky mouse if you lay the part on it's side. If you could give me the reasons you are thinking of I could check it to my print. Thanks for all the input so far, sorry for my lack of information.

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OK lets go with this then.
Are the parts good?

do they have a lot of scrap on this operation?

in all honesty if the parts are good and there is no scrap. whats the problem?

I think this is a case of shop supervisor with no or little machining experience. almost every shop has certain problem jobs its just the way stuff goes. personally I love problem jobs because they make you think of different ways of doing things and breaks the Menotti of everyday production.

you should really spell out the so call problems your having with the job. cause the way you explained it it could have absolutely nothing to do with the way its located.

Yes we have a lot of scrap, upwards of 10 percent or more. That is why I'm on here in the first place! We have green operators though so it's hard to tell how much of that is do to inexperienced. We are having all sorts of problems as well. Op 1 is pretty straight forward. It roughs out and finishes the ID and cuts a cylinder to the depth of the flange. They turn it around and cut's the bottom half of the cylinder first and shapes the flanges. The OD has a tolorence of +.001 and -.000. We use all of the tolorence to cut this part. It's out of round but there isn't a lot we can do about that. There is an allowance for a mismatch between the flange .020 above and below the flanges. I don't recall the GD & T tolorences and don't have the print in front of me at the moment but they appeared to be wide enough for some wiggle room on location. The datums are the top surface from op 1 and the ID for sure. Can't remember if there are anymore. It's made from steel, I believe 17-4. Know problems are a ding that randomly appears on the part. The remnant from cutting the flange isn't completely removed randomly. We had an issue with egging which I suggested we try to fix with a plug. They do not use torque to hold the part but I believe it's roughly 5 ft/lbs. of pressure to hold it. The mismatch between the flanges changes from part to part. Probably one or two more that I don't know as I really don't get the chance to personally run the freaking thing to feel it out myself! My problem? It's always been my opinion as I'm also kind of getting the impression from you guys, that locating on the movable jaw is done for a reason and not the norm. I personally have never really found a reason to do so in my 11 years on the job. I also can't get the powers that be to give me a reason they chose to do this. I also feel that the inconsistently from our process could potentially be from locating on a jaw that does not necessarily repeat. Heck, we could be fighting a lot of issues the hard way when transitioning to a more traditional approach could solve one or several of these issues.

You said you don’t know if they are probing? If parts are probed, it would be non issue. We do this when milling gear racks. It allows us to quickly and easily make the locating jaw whatever thickness.

However I have also used this method without probing. It can be done just fine for maybe +\- .01-.015”ish. There are a few jobs where I do this then mill OAL big adjust x and comp then run again. That is a waste of time, (you could create a quality fixed locating jaw and still make better on time depending on how many parts , but I’m not boss and I do as I’m told sometimes. ) but as long as you have the tolerance, or a probe there’s no reason it wouldn’t work that I have seen.

Are there features from the first op that are “timed” to match up with features from the second op? Would these relationships between the features of the first and second op require this sort of unconventional locating? Sounds like it may not. There are hundreds of different ways to hold a part. Doing it poorly just because “it’s always been done that way” is a horrible excuse for being lazy.
All this aside, if the boss says run it how it is, you are left with little choice. No matter how bad it sucks..
If you really want to change the boss’s mind, put the cost of your 10% scrap rate on paper. Then show him what re-fixturing will cost and how much it will save. It’s hard to argue with good numbers and a thorough comparison

I'm a job shop Machinist, so throw some salt over your shoulder. I have done some very interesting things to get good parts off. My list of interesting and sometimes stupid things is too long to list here. But if an outside person saw it, they might be a little critical and afraid too. I have been critical of set-ups that seemed strange to me, and was given the leeway to redo it, and came full circle in the end to the exact same strangeness that I was originally critical of. Point is be wary of your criticism of others before you know the whole story.

R